Arteriosclerosis is a disease that is characterized by a thickening and hardening of regions of an arterial wall. A particular type of arteriosclerosis is atherosclerosis, which affects the large arteries and is often the basis for coronary artery disease, aortic aneurysm, arterial disease of the lower extremities, and cerebrovascular disease. Atherosclerosis is characterized by the formation of fibrous plaques that contain a large number of smooth muscle cells, macrophages, collagen, extracellular lipid, and necrotic cell debris. The accumulation of material in a fibrous plaque results in narrowing of the blood vessel lumen which, in turn, restricts arterial blood flow. When the fibrous plaques become sufficiently large to block blood flow completely, the organs that are supplied by the artery undergo ischemia and necrosis. The accumulation of fibrous plaques also weakens the artery, an event which frequently results in rupture of the intima, aneurysm and hemorrhage. Moreover, fragments of the fibrous plaque may detach and form arterial emboli that can precipitate an aortic aneurysm or arterial disease of the lower extremities.
To date, the most frequently used methods for treating atherosclerosis include surgical procedures, drug therapies, and combinations of the foregoing. In general, the drug therapies for treating atherosclerosis are designed to prevent or reduce the accumulation of plaque material. For example, drugs such as diuretics, anti-adrenergic agents, vasodilators, angiotensin-converting enzyme inhibitors, renin inhibitors, and calcium channel antagonists have been used to treat conditions such as hypertension, hyperlipidemia, and hypercholesterolemia, which contribute to the development of atherosclerosis. Surgical methods for treating atherosclerosis include coronary bypass surgery, atherectomy, laser procedures, ultrasonic procedures, and balloon angioplasty. Such methods involve significant risk (e.g., of infection, death) to the patient and, even if successful, fibrous plaque formation frequently occurs at the site of vascular anastomoses, causing reclusion of the surgically-treated vessel.
Balloon angioplasty frequently results in injury to the blood vessel wall. Such vascular injury has been shown to induce proliferation and apoptosis in vascular smooth muscle cells (VSMCs), with the relative amounts of cell proliferation and apoptosis ultimately determining the size of the injury-induced lesion. Although peptide growth factors, receptors and their associated intracellular signaling pathways have been extensively studied in vascular smooth muscle cells (VSMCs), little is known about the VSMC nuclear factors that integrate these signals and initiate the regulatory cascades that determine whether a cell will proliferate, alter its state of differentiation, or undergo apoptosis.
Apoptosis is a cell death pathway that is highly conserved throughout evolution (Ameisen J C, et al., Science 1996; 272:1278-1279). Apoptosis is characterized by membrane blebbing and retention of its integrity, cellular and cytoplasmic shrinkage, and chromosomal fragmentation and condensation, endonuclease activation resulting in the characteristic 180 bp DNA ladder (Yang E and Korsmeyer S J, Blood 1996; 88:386-401). A number of stresses can induce apoptosis in vitro and in vivo, including the administration of glucocorticoids, removal of hormones, chemotherapy, mechanical injury, and DNA damage. Apoptosis is also induced by aberrant cell cycle activity, and can be triggered in cells that express the Fas receptor following activation of the Fas receptor by its natural binding partner, the Fas ligand. Cells expressing the Fas ligand (FasL) bind to cells that express the Fas receptor and thereby initiate a cascade that results in apoptosis (Nagata S and Golstein P, Science 1995; 267:1449-1456).
The Fas ligand is expressed in cytotoxic T lymphocytes and in immune privileged tissues such as the eye and testes. Recently, tumors have been reported to express Fas ligand, presumably, to allow tumor cells to protect themselves from cytotoxic T lymphocytes by inducing apoptosis in these cells (Hahne M et al., Science 1996; 274:1363-1366; Strand S et al., Nature Med. 1996; 2:1361-1366; O'Connell J et al., J. Exp. Med. 1996; 184:1075-1082). Several patents disclose the use of the Fas ligand/Fas receptor system for inducing apoptosis in lymphocytes and, thereby, harnessing the ability of these natural molecules to suppress lymphocyte-mediated immune responses such as autoimmune conditions and tissue rejection. For example, PCT Application no. PCT/US95/06742 ("Use of Fas Ligand to Suppress Lymphocyte-mediated Immune Responses", publication no. WO 95/32627) reports that intact and soluble mouse and human Fas ligand polypeptides and/or genes encoding such polypeptides, may be provided to a recipient mammal to suppress T-lymphocyte-mediated transplant or graft rejection. According to WO 95/32627, the compounds are also effective in suppressing and preventing lymphocyte-mediated primary disease, such as juvenile diabetes, and primary disease re-occurrence by, for example, introducing into a mammal a cell which expresses the Fas ligand.
In view of the foregoing, a need still exists to better understanding the molecular processes underlying injury-induced vascular smooth muscle cell proliferation and apoptosis, and to develop improved drug therapies to replace or supplement the existing methods for treating atherosclerosis and related conditions that are mediated by excessive smooth muscle cell proliferation. Preferably, such drug therapies would be designed to reduce or prevent plaque formation at its earliest stages.